Question 13131
Consider the length of the track. It remains the same regardless of what the width of the lane is. So the only change is the length of the curve in the track (the semicircle). If the length of the track was 0, all that would be left is a circle with a diameter of d. The length (circumference) is {{{PI}}}d.
Increasing the width of the track by 1.25 would increase the diameter of the dircle by 2.5. Now we have two equations: 
{{{c[1]=PI(d)}}} and {{{c[2]=PI(d+2.5)}}} The difference between these two circumferences is the increase in the track length from the inside to the outside of the lane. That is: {{{increase=PI(d+2.5) - PI(d)=PI(2.5)}}}
So the length of the track on the outside of the lane is {{{400+2.5(PI)}}}